This invention relates to a limb blood flowmeter for measuring the blood flow rate in human limb segments.
For example, in the case of using an artificial kidney, the blood is dialyzed by a hemodialyzer. In such a case, the measurement of the blood flow rate is indispensable to the determination of the period time for hemodialysis, that is, the time for each application of blood of substantially the whole body to the hemodialyzer. One method that has usually been employed in conventional hemodialyzers for the measurement of the blood flow rate is to make transparent a blood flow path between the hemodialyzer and the human body, form a bubble in the path at a certain place and measure the time for the passage of the bubble for a predetermined distance in the path, thereby to measure the blood flow rate. However, such a method is very troublesome, and the bubble in the blood entails a danger to the patient and, on top of that, the measurement obtained is relatively inaccurate.
In view of the abovesaid defects, there is a strong demand for means for accurately measuring the blood flow rate in the human body by a non-invasive method. One method for measuring blood flow rate or blood volume change non-invasively is the venous occlusion method. With this method, an occluding pressure cuff is wrapped around a limb such as an arm or leg to occlude the venous return and hence cause an increase in the tissue volume in the limb by the arterial inflow, and the increased tissue volume is measured to detect the blood flow rate. Thus the blood flow rate can be measured non-invasively without taking out a blood vessel for directly measuring the blood flow rate. This measurement is carried out in the following manner:- For example, an arm is immersed in water or like liquid contained in a measuring chamber, and the venous return is stopped, with the arm and the chamber held liquid-tight therebetween. An increase in the tissue volume of the arm by the arterial inflow is detected from the quantity of liquid which is caused to overflow by the arterial inflow, and then the blood volume flow is measured from the amount of tissue volume thus increased. However, a change in condition of the human limb due to the liquid temperature change during the measurement introduces an error in measurement. Accordingly the liquid temperature must be kept constant, and its control is complicated and, further, when the arm is immersed in the liquid for a long period of time, as mentioned above, the blood flow rate cannot be measured repeatedly and continuously.
A method that has been proposed for measurement of the ventricular stroke volume by measuring impedance changes based on ventricular systol is impedance plethysmography. This is set forth, for instance, in Medical Physics, Vol. II, Year Book 736/743 (1950), J. Nyboer, "Plethysmograph: Impedance", Aerospace Med. Vol. 37, 1208/1212 (1966), W. G. Kubicek et al, "Development and Evaluation of an Impedance Cardiac Output System" and so on. This method is to supply a high-frequency, very small current to a limb segment and measure the limb blood flow from a change in the electrical impedance of the limb segment caused by the venous occlusion. This method enables non-invasive and continuous measurement of the blood flow, but the impedance variation by a change in the blood volume is affected by the initial impedance value of the segment to be examined and does not coincide accurately with the actual change in the volume. Consequently the impedance variation is measured inclusive of the electrical characteristics of other tissues than that of the region desired to be examined, therefore the abovesaid method is defective in theory and in the accuracy of measurement.
Further, in his thesis submitted to the Faculty of the Graduate School of the University of Minnesota, 1965, "Cardiac Output Determinations Using Impedance Plethysmography", R. P. Patterson made a theoretical proposal of utilizing admittance for measuring the ventricular stroke volume.
An object of this invention is to provide a limb blood flowmeter which enables non-invasive, continuous and accurate measurement of the limb blood flow rate.
Another object of this invention is to provide a limb blood flowmeter which enables accurate measurement of the limb blood flow rate regardless of the initial admittance value of the limb and without including the electrical characteristics of other tissues than that of the region to be examined.
Still another object of this invention is to provide a limb blood flowmeter using the admittance method which is capable of direct measurement of a change in the blood volume independently of the initial admittance value of the limb to be examined.